1. Field of the Invention
The present invention relates to an electronic gauge for checking linear dimensions of mechanical pieces, more specifically during machining on grinding machines, including an outer casing, at least a movable arm, a feeler coupled to the movable arm, resilient means adapted for urging the movable arm in a measurement direction, a position transducer for generating a signal responsive to the position of the movable arm, an electric retraction device including a first element consisting of an electromagnet and a second element consisting of an armature and adjustment means for setting the amount of retraction.
2. Description of the Prior Art
Conventional electronic gauges for application on grinding machines include one or two movable arms on which there are urged springs, or other equivalent devices, in order to bring the feelers, coupled to the movable arms, into contact with the surface of a piece, or, if the piece is not present, into a rest position.
In the case of two-feeler gauges, for inside and outside diameter checking, the rest positions correspond, respectively, to the maximum and minimum distance between the feelers.
These gauges are normally equipped with a "retraction"0 device, for retracting the movable arm, or arms, from the rest position to a retracted position, defined by mechanical positive stops, thus serving to prevent collision of the feelers against the piece to be checked, or against other obstacles, when the gauge is moved towards, or away from, the piece, by means of hydraulic slides or equivalent devices.
Retraction devices may be of different types (pneumatic, hydraulic, etc.), but the type generally preferred is the electrical type, because it is simpler, more practical and safe. In fact, a retraction device, e.g. of a pneumatic type, depends upon an external service, necessitating, for its operation, a pneumatic system, that has to use filtered air, and consequently additional equipment.
It is obvious that similar drawbacks, like lack of practicalness and safety, also affect hydraulic retraction systems.
Additionally, it could be required to adjust the amount of arm retraction depending, for example, on the geometrical characteristics of the piece to be checked.
More specifically, in the case of gauges for checking inside diameters during machining on a grinding machine, the amount of the retraction must not be too large in order to prevent accidental impacts of the feelers against the grinding wheel located inside the hole to be checked and occupying a considerable amount of the space defined by the hole.
An adjustable electrical retraction device, for example, is the one employed in the gauge for checking inside diameters disclosed in Italian Pat. No. 664.927, filed on Mar. 21, 1962. In this gauge an electromagnet attracts within its windings a core, ending at one of its ends with a conical-shaped head that, by moving and cooperating--through two rollers--with the movable arms, causes a mutual approach of the two feelers. The amplitude of this movement can be set by adjusting (by means of a screw knob) a positive stop that limits the amount of displacement of the core.
In Italian Pat. No. 965.618 filed on May 23, 1972, there is described another type of gauge for checking inside diameters, comprising two movable arms carrying two flat surface armatures that are attracted by two electromagnets supported by the gauge casing. The amount of retraction is set by rotating a disk having two slots which house two reference elements fixed to the armatures.
In general, owing to the limited space available on grinding machines, it is advisable that gauges have rather small dimensions. Moreover, if the electromagnets of the retraction device have relatively high power they can give rise to an excessive thermal dissipation that may cause drifts in the performance of the gauge.
At the same time, however, if the attraction force applied by the electromagnet is not sufficiently high, there can occur, as a consequence of grinding machine vibrations, inconveniences in the retraction device, for example the unwanted release of the movable arms.
The retraction devices of the gauges described in the two previously mentioned Italian Patents, necessitate, in order to ensure their safe operation, electromagnets having rather high power and involving, consequently, excessive thermal dissipations.
With reference to the gauge described in the first of the two Italian Patents this depends, on the one hand, on the fact that the retraction device has a structure based on the "sucking" of the core within the windings. Moreover, in both the gauges described in the formerly mentioned Italian Patents, the attraction force applied by the electromagnets, on the core and armature respectively, under a condition of retraction of the movable arms, depends on the position of the adjustment devices, i.e. on the position of the positive stop and of the disk with slots.
Consequently, in the two formerly mentioned gauges there is achieved a poor compromise among the contrasting requirements of having small dimensions and thermal dissipations, safe performance of the retraction devices, broad adjustment range of the amount of retraction.